Meiosis: (Cell Division
via Meiosis)
Background:
This follows the page about an introduction
to cell division.
Reminder: Meiosis, which is also referred
to as "reduction division", is the form
of division of the cell nucleus by which a cell divides
into four "daughter cells" each of which
has half**
of the number of chromosomes of the original cell.
Meiosis s only occurs in the "gametes"
- prior to the formation of sperm (in males) and ova
(in females).
**The cells return to having
the normal (called "diploid") number of
chromosomes after fertilization of the ova by the
sperm.
Note the key difference that mitosis leads to TWO daughter calls - identical to each other and to the original cell,
whereas
meiosis leads to FOUR "daughter
cells" - each of which has half of
the number of chromosomes of the original cell.
So, meiosis is the type of division of cell
nuclei that occurs specifically to produce the gametes
- which are mature sex cells, specifically the ovum
in the female and the spermatozoon
in the male - that are necessary for sexual
reproduction.
(
The complete production of gametes,
including both the cell division stage and the subsequent maturing
of the gametes is called gametogenesis.)
The Stages of the "Cell Cycle" for Reproductive
Cells
In all reproductive cells (that is, all cells relating
to the reproductive parts of the body)
the "cell cycle" consists of four periods - two "interphase"
periods in which the cell is not dividing, and two "meiotic"
phases in which the cell is dividing:
Sequence:
- Interphase I
- Meiotic Phase I, consisting of:
(a) Meiosis (Nuclear Division I) - Prophase, Metaphase,
Anaphase and Telophase, followed by
(b) Cytokinesis (Cytoplasmic
Division I).
- Interphase II
- Meiotic Phase II, consisting of:
(a) Meiosis (Nuclear Division II) - Prophase, Metaphase,
Anaphase and Telophase, followed by
(b) Cytokinesis (Cytoplasmic Division II).
Due to this longer sequence of events, meiosis is
the more complex of the two types of cell division.
Notes:
- Meiosis involves
two successive divisions (both of which include
division of the cell nucleus, followed by division of the
cell cytoplasm).
They are called the First
Meiotic Division and the Second
Meiotic Division.
- Each of these successive
cell divisions may be described in terms of four phases
- called prophase, metaphase, anaphase and telophase.
- The
first meiotic division is similar to mitosis and
the second meiotic division is the "reduction" stage,
which differs slightly according to whether male
or female gametes are to be produced.
- Meiosis enables
the exchange of genetic material between chromosomes.
This takes place during the
early prophase phase of the first meiotic division.
See also related pages about Cell
Structure, Introduction
to Cell Division, Mitosis
and the Diagram
of Mitosis.
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